1. Field of Invention
The present invention relates to an actuating device of an optical pickup head. More particularly, the present invention relates to an electromagnetic actuating device that makes magnetic flux lines be distributed evenly.
2. Related Art
When a compact disc is placed in an optical disc drive, the optical pickup head of the optical disc drive will move along a guide rail to read the data of the compact disc, and then transmit them to the chipset at the host terminal for signal processing. In this course, since the compact disc is rotating at a high speed, and the compact disc itself is not in a perfect round shape, the rails on the compact disc are easily runout. Therefore, the optical pickup head must have a quick-response actuator for quickly moving the objective lens of the optical pickup head to focus on the predetermined rail.
In order to make the objective lens accurately focus on the rail on the compact disc that is predetermined to be read/written, the optical pickup head must have three actuating modes: (1) focusing: accurately controlling the distance between the objective lens of the pickup head and the surface of the compact disc to make the focus of the laser beam fall on the rail; (2) tracking: moving the objective lens horizontally to make the focus of the laser beam fall on the center of the rail, without going beyond the rail, or falling on the neighboring rail; (3) inclining: since the aberration caused by the distortion of the compact disc makes the focus of the laser beam move, the incident angle of the laser beam must be changed through inclining objective lens, so as to adjust the aberration caused by the distortion.
Referring to FIG. 1 and FIG. 2, they show a conventional optical pickup head actuator. A plurality of coils 1c is disposed around the objective lens carrier 1b that supports an objective lens 1a. The objective lens carrier 1b suspends under the support of the metal line 1d, and the metal line 1d receives electric power from the circuit board 1e and transmits it to the coil 1c, such that the coil 1c generates an electromagnetic force to make an interaction with the magnet 1f, so as to drive the objective lens carrier 1b to move.
Both sides of the objective lens carrier 1b are directly clad and clamped by the magnet 1f, such that the magnet interacts with the coil 1c on the objective lens carrier 1b, so as to drive the objective lens carrier 1b to move. On the other aspect, in order to enable the objective lens carrier 1b to move in the other horizontal direction, magnetic flux lines are guided to enter the objective lens carrier 1b from different directions by means of a yoke 1g, and to interact with an effective region of the coil 1c in the other direction, so as to drive the objective lens carrier 1b to move in the other horizontal direction.
Referring to FIG. 3, it shows the change of the magnetic flux (Tesla) along a vertical direction Z that passes through the coil 1c. The magnetic flux lines guided by the yoke 1g are divergently distributed in a vertical direction. When the objective lens carrier 1b drives the coil 1c to change its position in the vertical direction, the magnetic flux of the magnetic flux lines that pass through the coil 1c will be significantly changed, such that the coil 1c drives the objective lens carrier 1b to change, and thus the expected displacement or moving speed cannot be achieved. If the magnetic fluxes that pass through the opposite coils 1c are not consistent, during the tracking operation, forces impinged to both sides of the objective lens carrier 1b are inconsistent, and if both of the forces do not pass the mass center of the objective lens carrier 1b, they will produce a force moment to make the objective lens carrier 1b be unexpectedly inclined.
The conventional solution to the unexpected inclination problem lies in adjusting the height of the magnet 1f, and adding a heavy chunk to the objective lens carrier 1b to adjust the position of the mass center, in order to make the force applied by the magnetic flux lines pass through the mass center of the objective lens carrier 1b, and thereby preventing the magnetic force from producing a force moment on the objective lens carrier 1b. However, through this method, the applied force of the magnetic flux lines cannot pass through the mass center after the objective lens carrier 1b makes a vertical movement, which causes the forces impinged on two sides of the objective lens carrier 1b be inconsistent, and thus, the force moment occurs.